Store meta-data with observations

[tomaszmrugalski]

We need to make the upload procedure more robust, so other artifacts can be recorded.

• Info about the current configuration. Specific use case: when looking back at historic observations, it's unclear which SDR was used, with or without LNA, what recipe was used, with what parameters etc. This all should be recorded with each observation
• There's HIRS instrument on NOAA sats and there are tools to decode them
• We should record more detailed logs (so we could investigate issues after observation takes place, including some historic data)
• we want to have waterfall generated

All those reasons point to the specific solution: the submit_obs code (and its server counterpart) needs to become more robust to handle other data types.

[tomaszmrugalski]

With #63 merged, this is now possible. There are some quirks (e.g. server won't generate thumbnail if there's no image file), but overall it's possible to send multiple files, they don't have to be PNG and there is flexible, extensible metadata mechanism for defining extra data.